Physiological Condition of Juvenile Wading Birds in Relation to Multiple Landscape Stressors in the Florida Everglades: Effects of Hydrology,Prey Availability,and Mercury Bioaccumulation

The physiological condition of juvenile birds can be influenced by multiple ecological stressors, and few studies have concurrently considered the effects of environmental contaminants in combination with ecological attributes that can influence foraging conditions and prey availability. Using three temporally distinct indices of physiological condition, we compared the physiological response of nestling great egrets (Ardea alba) and white ibises (Eudocimus albus) to changing prey availability, hydrology (water depth, recession rate), and mercury exposure in the Florida Everglades. We found that the physiological response of chicks varied between species and among environmental variables. Chick body condition (short-term index) and fecal corticosterone levels (medium-term) were influenced by wetland water depth, prey availability, region, and age, but not by mercury contamination. However, mercury exposure did influence heat shock protein 70 (HSP70) in egret chicks, indicating a longer-term physiological response to contamination. Our results indicate that the physiological condition of egret and ibis chicks were influenced by several environmental stressors, and the time frame of the effect may depend on the specialized foraging behavior of the adults provisioning the chicks.     

Cattle egrets are less able to cope with light refraction than are other herons

The majority of heron species (Aves, Ardeidae) forage on aquatic prey in shallow water. Prey detection, aiming and the beginning of the capture strikes are performed while the heron's eyes are above water. For most angles, as a result of air/water light refraction, the apparent image available to a heron is vertically displaced from the prey's real position. Herons must therefore correct for refraction. We tested the hypothesis that species that forage in aquatic habitats should be more able to correct for image disparity than those of terrestrial habitats. The ability of hand-reared herons of four species to capture stationary prey (fish) underwater (submerged) or in air (aerial) was tested. Three species (little egret Egretta garzetta, squacco heron Ardeola ralloides, and night heron Nycticorax nycticorax) normally forage in aquatic habitats while the fourth (cattle egret Bubulcus ibis) forages in terrestrial habitats. No individuals missed aerial prey. Success rates of little egrets and of squacco herons with submerged prey were high, while night herons became less successful with increased prey depth and/or distance. In cattle egrets, success rate was low and negatively correlated with prey depth. The observed interspecific differences may thus be related to (1) differential ability to correct for air/water light refraction and (2) the species' foraging behaviour. We suggest that cattle egrets are in the process of losing their ability to cope with submerged prey. Copyright 1999 The Association for the Study of Animal Behaviour.     

广西防城7种鹭类混群繁殖的空间生态位研究

2002年3~8月对广西防城万鹤山营巢的鹭类繁殖种群进行了调查研究,结果表明:万鹤山栖息有大白鹭(Egretta alba)、中白鹭(Egretta intermedia)、白鹭(Egretta garzetta)、黄嘴白鹭(Egretta eulophotes)、牛背鹭(Bubulcus ibis)、池鹭(Ardeda bacchus)、夜鹭(Nycticorax nycticorax)7种鹭类种群。除黄嘴白鹭为均匀分布之外,其他6种均为成群分布。鹭类在万鹤山上营巢繁殖具有明显的水平分布和垂直分布现象。对影响鹭类混群繁殖时空间分布的因素及各种群间的相互关系进行了探讨。     

TAXONOMIC STATUS OF

Association of Cattle Egrets Bubulcus ibis with large herbivores is well documented, but there are few records of their association with large birds. Here we describe the first-known records of foraging interactions between Shoebill Balaeniceps rex and Cattle Egrets. The observations were made at the Malagarasi-Muyovozi Ramsar Site in western Tanzania. Small flocks of egrets approached and foraged within 5 m of a Shoebill, which was sometimes forced to move from its hunting pool and by doing so it likely flushed more prey for egrets. Interactions occurred almost exclusively in the driest months, which suggested that prey were more difficult to locate by egrets during this period. The Shoebill inhabits inaccessible swamps and is a rare species with low density throughout its range. It is therefore possible that egret–Shoebill associations, in addition to being infrequent and highly seasonal, may have gone unnoticed.     

Naturally occurring secondary nutritional hyperparathyroidism in cattle egrets (Bubulcus ibis) from central Texas

Naturally occurring secondary nutritional hyperparathyroidism is described in the nestlings of two colonies of cattle egrets (Bubulcus ibis) from Central Texas (Bryan and San Antonio, Texas, USA). Nestlings from a third colony (Waco, Texas, USA) were collected in a subsequent year for comparison. Birds from the first two colonies consistently had severe osteopenia and associated curving deformities and folding fractures of their long bones. These birds also had reduced bone ash, increased osteoclasia, a marked decrease in osteoblast activity, variable lengthening and shortening of the hypertrophic zone of the epiphyseal cartilage, decreased and disorganized formation of new bone, and a marked hypertrophy and hyperplasia of the parathyroid glands as compared to birds collected from the third colony. Fibrous osteodystrophy was found in all of the birds from San Antonio and Bryan. Evidence of moderate to severe calcium deficiency was also identified in 33% of the cattle egrets collected from Waco. Gut contents of affected chicks contained predominately grasshoppers and crickets; vertebrate prey items were absent from the Bryan birds. Grasshoppers and crickets collected from fields frequented by the adult egrets in 1994 had 0.12-0.28% calcium and 0.76-0.81% phosphorus. Pooled grasshoppers and crickets collected during a subsequent wet early spring averaged 0.24% calcium and 0.65% phosphorus. Although the phosphorus content of the insect prey was adequate for growth, calcium was approximately one-third the minimum calcium requirement needed for growth for other species of birds. It was postulated that cattle egrets breeding in Central Texas have expanded their range into habitat that contains less vertebrate prey, and as a result, many nestling egrets are being fed diets that contain suboptimal calcium. Therefore, in years where vertebrate prey is scarce and forage for insect prey is reduced in calcium, nestling egrets are at risk for developing secondary nutritional hyperparathyroidism.     

Organochlorine pesticides, polychlorinated biphenyls, metals, and trace elements in waterbird eggs, Salton Sea, California, 2004

The Salton Sea is a highly eutrophic, hypersaline terminal lake that receives inflows primarily from agricultural drainages in the Imperial and Coachella valleys. Impending reductions in water inflow at Salton Sea may concentrate existing contaminants which have been a concern for many years, and result in higher exposure to birds. Thus, waterbird eggs were collected and analyzed in 2004 and compared with residue concentrations from earlier years; these data provide a base for future comparisons. Eggs from four waterbird species (black-crowned night-heron [Nycticorax nycticorax], great egret [Ardea alba], black-necked stilt [Himantopus mexicanus], and American avocet [Recurvirostra Americana]) were collected. Eggs were analyzed for organochlorine pesticides, polychlorinated biphenyls (PCBs), metals, and trace elements, with current results compared to those reported for eggs collected from the same species and others during 1985–1993. The two contaminants of primary concern were p,p′-DDE (DDE) and selenium. DDE concentrations in night-heron and great egret eggs collected from the northwest corner of Salton Sea (Whitewater River delta) decreased 91 and 95%, respectively, by 2004, with a concomitant increase in eggshell thickness for both species. Decreases in bird egg DDE levels paralleled those in tissues of tilapia (Oreochromis mossambicus × O. urolepis), an important prey species for herons and egrets. Despite most nests of night-herons and great egrets failing in 2004 due to predation, predicted reproductive effects based on DDE concentrations in eggs were low or negligible for these species. The 2004 DDE findings were in dramatic contrast to those in the past decade, and included an 81% decrease in black-necked stilt eggs, although concentrations were lower historically than those reported in night-herons and egrets. Selenium concentrations in black-necked stilt eggs from the southeast corner of Salton Sea (Davis Road) were similar in 1993 and 2004, with 4.5–7.6% of the clutches estimated to be selenium impaired during both time periods. Because of present selenium concentrations and future reduced water inflow, the stilt population is of special concern. Between 1992 and 1993 and 2004 selenium in night-heron and great egret eggs from the Whitewater River delta at the north end of the Sea decreased by 81 and 55%, respectively. None of the night-heron or egret eggs collected in 2004 contained selenium concentrations above the lowest reported effect concentration (6.0 μg/g dw). Reasons for selenium decreases in night-heron and egret eggs are unknown. Other contaminants evaluated in 2004 were all below known effect concentrations. However, in spite of generally low contaminant levels in 2004, the nesting populations of night-herons and great egrets at Salton Sea were greatly reduced from earlier years and snowy egrets (Egretta thula) were not found nesting. Other factors that include predation, reduced water level, diminished roost and nest sites, increased salinity, eutrophication, and reduced fish populations can certainly influence avian populations. Future monitoring, to validate predicted responses by birds, other organisms, and contaminant loadings associated with reduced water inflows, together with adaptive management should be the operational framework at the Salton Sea. Guest editor: S. H. Hurlbert The Salton Sea Centennial Symposium. Proceedings of a Symposium Celebrating a Century of Symbiosis Among Agriculture, Wildlife and People, 1905–2005, held in San Diego, California, USA, March 2005     

Do cattle egrets gain information from conspecifics when foraging?

Summary We examined whether individual cattle egrets (Bubulcus ibis) base their decisions of where to forage, and how long to stay in a patch, on the behavior of other flock members. Cattle egrets commonly forage in flocks associated with cattle and capture prey at higher rates when they do not share a cow with another egret. Foraging egrets provide cues of the location of prey and their success in capturing prey. Therefore, there is the possibility of information transfer between egrets in a flock. We predicted that egrets should only move to occupied patches when the resident was capturing enough prey that it is profitable for the invader to share the patch or take over the patch. However, egrets did not seem to decide where to forage based on neighbors' rates of energy intake, but rather on the presence or absence of conspecifics in a patch. We also predicted that an egret should remain in a patch until its rate of energy intake dropped to or below the average rate for other egrets within the flock. However, egrets that were foraging more efficiently than the average rate for the flock switched patches sooner than less efficient foragers. Egrets did not appear to increase foraging success by gaining information on patch quality from neighbors.     

Phosphorus and nitrogen in wetlands with and without egret colonies

Abstract Colonially nesting birds have been implicated in the destruction of their nest site vegetation, the most likely mechanism being through an overload or imbalance of soil nutrients. The numbers of egrets in coastal New South Wales colonies have increased rapidly in the second half of this century since cattle egrets (Ardeola ibis) were introduced. This species now nests colonially with three native species: great (Egretta alba), intermediate (Egretta intermedia) and little egrets (Egretta garzetta). Thus there is potential for more rapid degradation of the colony site. We measured the levels of phosphorus in surface sediments, and phosphorus and nitrogen in water at eight colony site wetlands, and compared each to four other wetlands in the same region where there were no egret colonies. We conclude that levels of both nutrients were elevated in colony wetlands. The implications of this result for death of colony site vegetation and the conservation of the three native egret species are briefly discussed.     

Interim Response of Wading Birds (Pelecaniformes and Ciconiiformes) and Waterfowl (Anseriformes) to the Kissimmee River Restoration Project,Florida, U.S.A

Success of the Kissimmee River Restoration Project will be evaluated in part by monitoring populations of wading birds (Pelecaniformes and Ciconiiformes) and waterfowl (Anseriformes). These two waterbird guilds were integral components of the pre‐channelization river–floodplain ecosystem, and both declined substantially following channelization. Restoration is expected to attract wading birds and waterfowl by reintroducing naturally fluctuating water levels, seasonal hydroperiods, and historic vegetation communities. Post‐construction aerial surveys (November 2001 to May 2008) within the Phase I restoration area indicate that the abundance and species richness of both wading birds and waterfowl have shown a positive restoration response thus far. Dry season abundance of aquatic wading birds and waterfowl has exceeded restoration expectations (≥30.6 birds/km² and ≥3.9 birds/km², respectively) each year since the completion of restoration Phase I in 2001. While there has been a significant positive restoration effect on waterfowl abundance, waterfowl species richness (n = 6) has not yet reached the restoration expectation of ≥13 species. Abundance of the terrestrial cattle egret (Bubulcus ibis), which increased dramatically after the majority of floodplain wetlands were converted to cattle pastures in the channelized system, has shown a significant negative response to restoration. It is anticipated that completion of the remaining phases of restoration (II/III), and implementation of the Kissimmee River Headwaters Revitalization water regulation schedule by 2019, will further increase and improve habitat for wading birds and waterfowl by reestablishing floodplain hydrology that more closely mimics historical conditions.     

Responses of Australian wading birds to a novel toxic prey type, the invasive cane toad Rhinella marina

The impact of invasive predators on native prey has attracted considerable scientific attention, whereas the reverse situation (invasive species being eaten by native predators) has been less frequently studied. Such interactions might affect invasion success; an invader that is readily consumed by native species may be less likely to flourish in its new range than one that is ignored by those taxa. Invasive cane toads (Rhinella marina) in Australia have fatally poisoned many native predators (e.g., marsupials, crocodiles, lizards) that attempt to ingest the toxic anurans, but birds are more resistant to toad toxins. We quantified prey preferences of four species of wading birds (Nankeen night heron, purple swamphen, pied heron, little egret) in the wild, by offering cane toads and alternative native prey items (total of 279 trays offered, 14 different combinations of prey types). All bird species tested preferred the native prey, avoiding both tadpole and metamorph cane toads. Avoidance of toads was strong enough to reduce foraging on native prey presented in combination with the toads, suggesting that the presence of cane toads could affect predator foraging tactics, and reduce the intensity of predation on native prey species found in association with toads.     

The feeding success of cattle egrets in flocks

The feeding rates of grouped (<1.5 m from conspecifics) and solo (>5 m from conspecifics) cattle egrets (Bubulcus ibis) in loose flocks away from cows were compared, to test the hypothesis that grouped cattle egrets benefit from feeding on prey flushed inadvertently by nearby conspecifics. The flock feeding rates were also compared to those of grouped and solo egrets near cows, to determine the effects of flock membership on feeding rates. Birds in flocks captured prey faster than those with cows, and tended to capture larger prey, but field observations and captive experiments failed to show that the feeding success of flock members was enhanced by the hypothesized ‘beater’ effect. Increases in prey density, however, always resulted in higher feeding rates, so some cattle egret groups may form in response to local concentrations of prey. Prey size may also play a role in group formation, because birds in the field tended to feed at greater distances from their neighbours when larger prey were captured, regardless of prey density. When small groups did form among cattle egrets feeding on relatively large prey, group members occasionally captured prey items that had been discovered by nearby conspecifics. This behaviour was not observed among birds in dense aggregations, which fed on small, highly abundant prey. These data indicate that there is a potential cost associated with feeding too near others unless the prey are relatively small and abundant.     

Facilitation between herons and smallmouth bass foraging on common prey

Understanding how multiple predators affect one another and their shared prey is an increasingly important goal for ecologists examining predator–prey dynamics and food-web structure. In a field experiment, we examined the outcome of interactions between terrestrial and freshwater predators foraging for the same prey in two temperate North American streams. We used a factorial design to examine the combined foraging effects of herons and smallmouth bass on striped shiners and central stonerollers. We found that there was facilitation between the two predators, resulting in risk-enhancement for the prey species, with particularly pronounced effects on the smallest (<70 mm) size classes. Facilitation is the least well-documented predator–predator interaction and has not been quantitatively demonstrated for freshwater and terrestrial predators. Our results indicate that bass may gain a net benefit from the presence of wading birds such as herons and egrets, and that concerns about the negative effects of birds on fish stocks through competition may be unwarranted.     

厦门大屿岛三种鹭的种群动态和营巢

对厦门大屿岛白鹭保护区的夜鹭（Nycticorax nycticorax）、白鹭（Eegrtta garzetta）和池鹭（Ardeola bacchus）进行种群动态和营巢观察研究。3种鸟的数量季节变化都很大,5～7月数量高峰期,每月总数都达5000只左右。5月底共5610只,3种鸟的比例分别为50.8%、32.0%和17.2%。4月开始大量在相思树梢上营巢,5月中至6月初巢数多达6000多个。结果表明,3种鸟都是厦门的繁殖鸟和留鸟。     

Effects of water level changes and wading bird abundance on the foraging behaviour of blacknecked storks Ephippiorhynchus asiaticus in Dudwa National Park, India

The effect of water level changes and wading birds' abundance on the foraging behaviour of the blacknecked stork (BNS)Ephippiorhynchus asiaticus was studied from January 1995 to June 1997 in Dudwa National Park, Uttar Pradesh. Our observations indicate that BNS territoriality increased as food levels became depleted, resulting in increased rates of aggression towards intruders. Chasing or aggression was more intense during the early period (February and March) than the late period (April, May and June). Most of (> 50%) the aggressive encounters were observed between 0600 and 1000 h of the day. Seventeen species (including BNS) were observed interacting with BNS, throughout the study period. Most interactions were with the spoonbill,Platalea leucorodia (67.4%), followed by the whitenecked stork,Ciconia episcopus (16.6%). The distance (while foraging) between BNS and other wading birds varied significantly (P < 0001) between years indicating that BNS and other water birds foraged at different water depths and thereby explored the wetlands fully. Spoonbills were chased often; the number varied from 1 to 43 birds. BNS occasionally accepted the presence of other wading birds, including spoonbills and started foraging amidst them. This led to successful foraging of BNS (solitary feeder). Other fish-eating bird species and their numbers also limited the food consumption of foraging BNS as they had to spend time chasing away the intruders. Availability of the preferred prey fish species,Heteropnestus fossilis, forced BNS to stay throughout the year in their respective territories. High (> 60 cm) water levels were not suitable for BNS even though the patch had high prey abundance.     

New primers for sex identification in the Chinese egret and other ardeid species

Using the universal P2/P8 primers, we were able to obtain the gene segments of chromo-helicase-DNA binding protein (CHD)-Z and CHD-W from ten species of ardeid birds including Chinese egret (Egretta eulophotes), little egret (E. garzetta), eastern reef egret (E. sacra), great egret (Ardea alba), grey heron (A. cinerea), Chinese pond-heron (Ardeola bacchus), cattle egret (Bubulcus ibis), black-crowned night-heron (Nycticorax nycticorax), cinnamon bittern (Ixobrychus cinnamomeus) and yellow bittern (I. sinensis). Based on conserved regions inside the P2/P8-derived sequences, we designed new PCR primers for sex identification in these ardeid species. Using agarose gel electrophoresis, the PCR products showed two bands for females (140 bp derived from CHD-W and the other 250 bp from CHD-ZW), whereas the males showed only the 250 bp band. The results indicated that our new primers could be used for accurate and convenient sex identification in ardeid species.     

How does the presence of a conspecific individual change the behavioral game that a predator plays with its prey?

Behavioral games predators play among themselves may have profound effects on behavioral games predators play with their prey. We studied the behavioral game between predators and prey within the framework of social foraging among predators. We tested how conspecific interactions among predators (little egret) change the predator–prey behavioral game and foraging success. To do so, we examined foraging behavior of egrets alone and in pairs (male and female) in a specially designed aviary consisting of three equally spaced pools with identical initial prey (comet goldfish) densities. Each pool was comprised of a risky microhabitat, rich with food, and a safe microhabitat with no food, forcing the fish to trade off food and safety. When faced with two versus one egret, we found that fish significantly reduced activity in the risky habitat. Egrets in pairs suffered reduced foraging success (negative intraspecific density dependence) and responded to fish behavior and to their conspecific by changing their visiting regime at the different pools—having shorter, more frequent visits. The time egret spent on each visit allowed them to match their long-term capture success rate across the environment to their capture success rate in the pool, which satisfies one aspect of optimality. Overall, egrets in pairs allocated more time for foraging and changed their foraging tactics to focus more on fish under cover and fish ‘peeping’ out from their shelter. These results suggest that both prey and predator show behavioral flexibility and can adjust to changing conditions as needed in this foraging game.     

Capturing American White Ibises in urban South Florida using two novel techniques

As many wildlife species, including wading birds, adapt to anthropogenic landscapes and, in some cases, exhibit altered behaviors, studies that involve capturing birds may require new methods better suited for use in urban areas and to accommodate altered animal behavior. We developed two novel techniques, a leg lasso and flip net, for capturing American White Ibises (Eudocimus albus) in urban environments in southern Florida, and also used a traditional technique (mist‐nets) in non‐urban wetland habitats. The flip net and leg lasso were developed to capture White Ibises habituated to the presence of humans. Ibises were captured in urban and wetland environments from October 2015 to August 2017 in Palm Beach, Broward, and Lee counties, Florida. We captured 6.0 ± 13.5 ibis/h with the flip net, 1.6 ± 0.8 ibis/h with the leg lasso, and 0.5 ± 2.6 ibis/h with mist‐nets. We captured larger (higher mass to tarsus length ratio) birds using the flip net and leg lasso than using mist‐nets, and captured more males with leg lassos than with other two techniques. The novel techniques we used are efficient, cost effective, easy to use, and also potentially useful for capturing other species of birds. Leg lassos and flip nets are also safe to use in populated areas for both birds and humans.     

Aquatic prey switching and urban foraging by the White Ibis Eudocimus albus are determined by wetland hydrological conditions

Prey availability is known to limit reproduction of some species of nesting birds, but identifying the primary prey types of a species with a flexible diet can be challenging. For the White Ibis Eudocimus albus, a tactile feeding, medium‐sized wading bird, nestling prey composition is suggested to depend on landscape water depths/availability of foraging habitat at the time of nesting and on historical drying events affecting prey production. We collected and compared inter‐ and intra‐annual diet variation of White Ibis chicks reared in the Everglades over two years that were independently identified as being relatively good (2006) and poor (2007) nesting seasons. We collected 127 nestling boluses and analysed the temporal variation in biomass of eight functional prey groups using multivariate techniques. The boluses from 2006 in the central Everglades were dominated by fish, but in 2007, after fish had been reduced by the previous year of drying, the boluses from the same region were more variable and dominated by garbage (i.e. scavenging). Analysis of five different collections taken from a different colony in the northern Everglades indicated that boluses were characterized by crayfish and had fewer fish or less garbage when landscape water depths were relatively higher and more preferred habitat was available. At lower landscape water depths in 2007 the bolus composition shifted away from crayfish towards small fish and urban food (terrestrial insects and garbage). Our results support the suggestion of depth‐dependent diets; prey composition depends on the current landscape water levels around the colonies, and also suggests that previous drying events can lead to increased reliance on alternative food sources. White Ibis partially compensated for unavailable aquatic prey with alternative urban foods, but their nesting success appears to have suffered.     

浙江五峙山列岛黄嘴白鹭的巢位选择研究

黄嘴白鹭(Egretta eulophotes)是全球性受威胁鸟类。2005年7月, 我们在浙江五峙山列岛对黄嘴白鹭的巢位选择进行了研究, 共测量了74个巢。研究发现, 在0.99 hm2的馒头山小岛上, 黄嘴白鹭的巢呈聚集性分布, 平均巢间距为119.6 cm, 最小巢间距为15 cm。黄嘴白鹭对巢位具有明显的选择性, 优先选择在灌丛, 尤其是那些相对较高的灌丛(高于50 cm)下营巢, 其次是较低的灌木(低于64 cm)内部和草丛上。多数巢存在遮蔽物。避风和遮荫是黄嘴白鹭巢位选择的主要要求。台风是决定五峙山列岛黄嘴白鹭巢位选择的最主要因素, 而高灌木的数量直接影响五峙山列岛黄嘴白鹭的繁殖成功和今后的资源状况。     

Response of the waterbird community to floating pennywort (Hydrocotyle ranunculoides) cover at Ngamo dam,Antelope Park,Zimbabwe.

Little is known on the influence of invasive aquatic weeds on afro‐tropical waterbird communities. We used bird counts in sites of varying floating pennywort Hydrocotyle ranunculoides cover to explore the relationship between the weed and the waterbird community dynamics at Ngamo dam, Antelope Park, Zimbabwe. Waterbird communities in low‐to‐medium weed cover sites were more diverse and abundant compared to sites of high weed cover. In addition to supporting birds such as African Jacana which are able to forage within dense aquatic plants, high weed cover sites were associated with birds whose diets include invertebrates and fish which are likely more abundant and diverse in these sites. In contrast, low‐to‐medium weed cover sites were associated with bird species such as Common Moorhen, Great Egret, Pied Kingfisher and African Fish Eagle which require accessible open water and forage for diving, swimming or wading. The increased bird abundance in low‐to‐medium weed cover sites also likely increases prey for predatory birds. Thus, the negative changes in the waterbird community composition, abundance and diversity in response to increasing floating pennywort cover reflects species‐specific tolerances to floating pennywort and its influence on accessible open water, foraging sites and prey availability.